Pyridine, C.sub.5 H.sub.5 N, is a six-membered heterocyclic compound containing one nitrogen atom. Pyridine is miscible in all proportions with water and its boiling point is 115.3.degree. C. Pyridine forms an azeotrope with water at 93.6.degree. C., containing 41 wt-% water. Pyridine derivatives are for example alkylpyridines, vinylpyridines, halopyridines, aminopyridines, pyridinols and pyridyl alcohols. Alkylpyridines are for example 2-methylpyridine, 3-methylpyridine, 4-methylpyridine, 2.6-dimethylipyridine, 3.5-dimethylipyridine and 2-ethyl-2-methylipyridine. Pyridine and pyridine derivatives are used as starting material for pharmaceutical and agrochemicals. Pyridine compounds are also used as a solvent and as catalysts.
The reaction of aldehydes or ketones with ammonia is the most general industrial synthetic reaction for the manufacture of pyridine bases. The reaction is usually carried out at 350-500.degree. C. and atmospheric pressure in the presence of a catalyst. A schematic flow sheet of pyridine and methylpyridine production is presented in literature.sup.1. Ammonia, acetaldehyde and formaldehyde are fed to a catalyst containing reactor, where pyridine, alkylpyridine, water and hydrogen are formed. The reaction mixture is separated from ammonia and hydrogen by a collector, after which pyridine and methylpyridines are extracted from aqueous solution with suitable solvent, e.g. with benzene. The solvent is separated from the pyridines by distillation and recycled to the extraction column. Pyridine and alkylpyridines are isolated in continuous distillation columns.
Pyridine or pyridine derivative containing waste waters are formed in the industry. Because pyridine is very bioactive, it must be removed from water with a suitable method.
It is known that hydrocarbons and oxygenated hydrocarbons, such as alcohols, esters, organic acids, aldehydes, ketones or phenols, can be extracted from aqueous solutions by using near-critical or supercritical state solvent, such as carbon dioxide, ethane or ethene (U.S. Pat. No. 4,349,415). It is also known that the extraction of dioxane, acetone, formamide. N.N-dimethylformamide and ethyleneglycol have been tried.sup.2. The above mentioned compounds are neutral or acidic, and there is no acid-base interaction between the solvent and the solute.
It is difficult to extract nitrogen containing organic compounds from water with carbon dioxide for two reasons. First, many of the nitrogen containing compounds are very soluble in water, and the distribution coefficient is very low. For example, the distribution coefficient of formamide in CO.sub.2 -water formamide systems is only 0.001 (weight bases), which is too small to allow effective extraction.sup.3. Second, the pH of water will decrease to approximately 3.5 as high density carbon dioxide is introduced, and any reasonably basic compound will protonate, increasing its solubility in water and decreasing solubility in carbon dioxide. Protonated compounds are not expected to be extractable with non-polar carbon dioxide solvent. Further, if the compound is a strong base, it will react with acidic carbon dioxide. It is known from the literature that 2.5-dimethylpyridine has been tried to extract from water with supercritical carbon dioxide.sup.4. In these experiments 2.5-dimethylpyridine could not be extracted from dilute aqueous solution with supercritical carbon dioxide.